Ultrastructural Analysis of Spermiogenesis in Segregation Distorter Males of DROSOPHILA MELANOGASTER: The Homozygotes

We have studied spermiogenesis at the ultrastructural level in males of genotype SD(NH)-2/SD-72, which are nearly sterile owing to the dysfunction of virtually all of their sperm. Ultrastructural aspects of spermiogenesis in these homozygous SD males are qualitatively similar to those found among dysfunctional sperm produced by heterozygous SD males. In particular, chromatin condensation and/or compaction has been found to be abnormal. However, major quantitative differences have been noted. Most of the dysfunctional sperm in SD(NH)-2/SD-72 males are individualized and coiled. Then, the sperm evidently undergo degeneration, as few mature sperm can be found in the seminal vesicle. The relevance of these findings to the mechanism leading to near sterility in homozygous SD males is discussed.     

On the Components of Segregation Distortion in DROSOPHILA MELANOGASTER

The segregation distorter (SD) complex is a naturally occurring meiotic drive system with the property that males heterozygous for an SD-bearing chromosome 2 and an SD⁺-bearing homolog transmit the SD-bearing chromosome almost exclusively. This distorted segregation is the consequence of an induced dysfunction of those sperm that receive the SD⁺ homolog. From previous studies, two loci have been implicated in this phenomenon: the Sd locus which is required to produce distortion, and the Responder (Rsp) locus that is the site at which Sd acts. There are two allelic alternatives of Rsp—sensitive (Rsp^sens) and insensitive (Rsp^ins); a chromosome carrying Rsp^ins is not distorted by SD. In the present study, the function and location of each of these elements was examined by a genetic and cytological characterization of X-ray-induced mutations at each locus. The results indicate the following: (1) the Rsp locus is located in the proximal heterochromatin of 2R; (2) a deletion for the Rsp locus renders a chromosome insensitive to distortion; (3) the Sd locus is located to the left of pr (2-54.5), in the region from 37D2-D7 to 38A6-B2 of the salivary chromosome map; (4) an SD chromosome deleted for Sd loses its ability to distort; (5) there is another important component of the SD system, E(SD), in or near the proximal heterochromatin of 2L, that behaves as a strong enhancer of distortion. The results of these studies allow a reinterpretation of results from earlier analyses of the SD system and serve to limit the possible mechanisms to account for segregation distortion.     

Environmental effects on Segregation-distorter in Drosophila melanogaster: Irradiation of SD-72 at the onset of spermatogenesis

To ascertain the effect of irradiation at the onset of spermatogenesis on Segregation-distorter, irradiation treatments were administered to very late third instar male SD larvae. Part of the experimental and control groups were stored. The first four daily broods from unstored irradiated males exhibited depressed k values and aberration products. The percentage of aberrations observed per sperm batch is inversely correlated with the k values exhibited. The SD mechanism appeared to be most sensitive to irradiation prior to or at Prophase I. Using aberration products as an index, stored males transfer few sperm which were in meiotic or premeiotic stages at the time of treatment. The induction of these aberrations affected the action of the SD mechanism very little. The SD-bearing exceptional chromosomes were 0.9 of the total exceptions recovered. Lack of mating activity depressed the drive of SD in both irradiated and control groups. Prolongation of spermiogenesis may allow recovery of SD+ gametes which were rendered temporarily dysfunctional.This work was completed at Michigan State University during the tenure of a National Institutes of Health Fellowship and partially supported by a grant to Dr. Armon F. Yanders from the U.S. Atomic Energy Commission (Contract AT (11-1) 1933)     

Additive Effects of Multiple SEGREGATION DISTORTER ( SD) Chromosomes on Sperm Dysfunction in DROSOPHILA MELANOGASTER

A portion of the Segregation distorter (SD) chromosome, including both the Sd and E(SD) loci, has been moved by insertional translocation from SD Roma into Y^L . This Dp(2;Y)SD chromosome shows a negligible reduction in its ability to cause dysfunction of Rsp ^s-bearing sperm when compared to the parent SD chromosome, suggesting that SD can still act effectively, even when removed from its normal second chromosome milieu, and that its activity level does not depend on pairing with a normal autosomal homologue. Male genotypes have been constructed using this Dp(2;Y)SD along with a standard SD chromosome (either SD Roma or R( SD-36)-1^bw) and a third chromosome suppressor of SD (TM6) in all possible three-way combinations. The observed level of SD-mediated dysfunction in each case is most compatible with a model that assumes that all SD elements act additively (in terms of M, the probit transformation of the probability of sperm dysfunction), rather than multiplicatively. The additive action of SD elements contrasts with the independent response to SD activity exhibited by multiple Rsp^s copies.     

Segregation distorter in D. melanogaster males: An effect of female genotype on recovery

Summary Segregation Distorter (SD) is a gene affecting sperm recovery in Drosophila melanogaster. In a cross SD/SD ⁺ x SD⁺, the proportion of SD/SD ⁺ zygotes recovered (k) is larger than the 0.50 expected. Previous investigations have shown that the relative recovery of SD- and SD ⁺-bearing sperm is determined at or before the meiotic divisions. Evidence is presented here that k is influenced by the genotype of the female parent. It is suggested that the SD gene product causes a difference in the enzymatic or structural complement of the SD- and SD ⁺-bearing sperm which largely determines their relative functionality. The effect of the female genotype on recovery is interpreted as an interaction between this physiological difference and the environment of the female reproductive tract.Adapted from a dissertation presented by the senior author in partial fulfillment of the degree of Doctor of Philosophy. This investigation was supported by PHS Training Grant No. GM 00337 and PHS Research Grant No. GM 12334 from the National Institute of General Medical Sciences.     

The relationship between first and second chromosome segregation ratios from Drosophila melanogaster males bearing Segregation Distorter

Summary Drosophila melanogaster males heterozygous for the second chromosome locus Segregation Distorter preferentially transmit this chromosome to their progeny due to a dysfunctioning of SD ⁺-bearing sperm. SD males with a normal sex chromosome constitution produce more females than males among SD ⁺ progeny. This report shows that this unequal recovery of sexes is enhanced from XY/Y; SD/SD ⁺ males and enhanced still further from XY/O; SD/SD ⁺ males. It is argued that the probability that a SD ⁺-bearing sperm will dysfunction is related to its sex chromsome complement, with the relative probabilities of dysfunction ranked O> Y> X> XY. It is shown that a modified probit analysis accounts for the relationship between sex ratio and second chromosome segregation frequency for all paternal genotypes. Finally, SD/SD ⁺ males show no increase in sex chromosome nondisjunction with respect to a control.R. E. Denell was supported by U.S.P.H.S. Training Grant No. GM00337 and by a U.S.P.H.S. Postdoctoral Fellowship; George L. Gabor Miklos was supported by A.E.C. Contract No. AT (04-3)-34 PA150.     

On the Components of Segregation Distortion in DROSOPHILA MELANOGASTER. II. Deletion Mapping and Dosage Analysis of the SD Locus

Segregation distorter (SD) chromosomes are preferentially transmitted to offspring from heterozygous SD/SD⁺ males owing to the induced dysfunction of the SD⁺-bearing sperm. This phenomenon involves at least two major loci: the Sd locus whose presence is necessary for distortion to occur and the Rsp locus which acts as the site of Sd action. Several additional loci on SD chromosomes enhance distortion.—In a previous study deletions were used to map the Sd locus and to determine some of its properties. We have extended this analysis with the isolation and characterization of 14 new deletions in the Sd region. From our results we conclude (1) SD chromosomes contain a single Sd locus located in region 37D2-6 of the salivary gland chromosome map. Deletion of this locus in any of three SD chromosomes now studied results in complete loss of ability to distort a sensitive chromosome; (2) the reduced male fecundity observed in many homozygous SD or SD_i/SD_j combinations is due at least in part to the action of the Sd locus. The fecundity of these males can be substantially increased by deletion of one Sd locus. Thus, it is the presence of two doses of Sd rather than the absence of Sd⁺ that produces the lowered male fecundity in SD homozygotes; (3) Sd behaves as a neomorph, whereas Sd⁺, if it exists at all, is amorphic with respect to segregation distortion; (4) these results support a model in which the Sd product is made in limiting amounts and the interaction of this product with the Rsp locus causes sperm dysfunction. The Sd product appears to act preferentially at Rsp^s (sensitive-Responder) but may also act at Rspⁱ (insensitive-Responder).     

Genetic Dissection of Segregation Distortion. I. Suicide Combinations of SD Genes

Two major loci in the Tft–cn region of an SD chromosome have been separated by recombination and identified. The allele at the left-hand locus on an SD chromosome is called Sd; the allele at the right-hand locus is called Rsp. Both Sd and Rsp are necessary to bring about a distortion of the segregation ratio in heterozygous SD males, although the particular degree of distortion exhibited by an SD chromosome is influenced by the constellation of polygenic modifiers of SD in the genome. The coupling phase of the alleles, Sd Rsp/Sd⁺Rsp⁺, produces about 89-90% of Sd Resp-bearing progeny. The repulsion phase, Sd Rsp⁺/Sd⁺ Rsp, produces 10-20% of Sd Rsp⁺-bearing progeny. No coupling-repulsion effects between Sd and Rsp are apparent.     

Distinct spermiogenic phenotypes underlie sperm elimination in the Segregation Distorter meiotic drive system

Segregation Distorter (SD) is a male meiotic drive system in Drosophila melanogaster. Males heterozygous for a selfish SD chromosome rarely transmit the homologous SD⁺ chromosome. It is well established that distortion results from an interaction between Sd, the primary distorting locus on the SD chromosome and its target, a satellite DNA called Rsp, on the SD⁺ chromosome. However, the molecular and cellular mechanisms leading to post-meiotic SD⁺ sperm elimination remain unclear. Here we show that SD/SD⁺ males of different genotypes but with similarly strong degrees of distortion have distinct spermiogenic phenotypes. In some genotypes, SD⁺ spermatids fail to fully incorporate protamines after the removal of histones, and degenerate during the individualization stage of spermiogenesis. In contrast, in other SD/SD⁺ genotypes, protamine incorporation appears less disturbed, yet spermatid nuclei are abnormally compacted, and mature sperm nuclei are eventually released in the seminal vesicle. Our analyses of different SD⁺ chromosomes suggest that the severity of the spermiogenic defects associates with the copy number of the Rsp satellite. We propose that when Rsp copy number is very high (> 2000), spermatid nuclear compaction defects reach a threshold that triggers a checkpoint controlling sperm chromatin quality to eliminate abnormal spermatids during individualization.     

Large-Scale Selective Sweep among Segregation Distorter Chromosomes in African Populations of Drosophila melanogaster

Segregation Distorter (SD) is a selfish, coadapted gene complex on chromosome 2 of Drosophila melanogaster that strongly distorts Mendelian transmission; heterozygous SD/SD⁺ males sire almost exclusively SD-bearing progeny. Fifty years of genetic, molecular, and theory work have made SD one of the best-characterized meiotic drive systems, but surprisingly the details of its evolutionary origins and population dynamics remain unclear. Earlier analyses suggested that the SD system arose recently in the Mediterranean basin and then spread to a low, stable equilibrium frequency (1–5%) in most natural populations worldwide. In this report, we show, first, that SD chromosomes occur in populations in sub-Saharan Africa, the ancestral range of D. melanogaster, at a similarly low frequency (~2%), providing evidence for the robustness of its equilibrium frequency but raising doubts about the Mediterranean-origins hypothesis. Second, our genetic analyses reveal two kinds of SD chromosomes in Africa: inversion-free SD chromosomes with little or no transmission advantage; and an African-endemic inversion-bearing SD chromosome, SD-Mal, with a perfect transmission advantage. Third, our population genetic analyses show that SD-Mal chromosomes swept across the African continent very recently, causing linkage disequilibrium and an absence of variability over 39% of the length of the second chromosome. Thus, despite a seemingly stable equilibrium frequency, SD chromosomes continue to evolve, to compete with one another, or evade suppressors in the genome.     

The Influence of Chromosome Content on the Size and Shape of Sperm Heads in DROSOPHILA MELANOGASTER and the Demonstration of Chromosome Loss during Spermiogenesis

The volumes of sperm heads were estimated from three-dimensional reconstructions of serially sectioned bundles of nearly mature spermatid nuclei. Cysts from males in which all sperm are expected to have comparable amounts of chromatin (X/Y and In(3LR)/+) show unimodal frequency distributions of nuclear volumes, whereas cysts from males in which meiotic segregation is expected to deliver unequal amounts of chromatin material to spermatid nuclei show two (XY/O and XY/Y) or more (T(2;3)/⁺ and C(2L);C(2R)) modes. The mean volumes of the subpopulations in these cases are related in the same proportions as the metaphase lengths of their chromosomal complements. Thus the volumes of sperm nuclei are proportional to their DNA content. Sperm head shape, on the other hand, does not appear to be very sensitive to chromosomal constitution, as heads of different size do not vary greatly in shape.—The numbers of sperm heads in the various size classes in a cyst depart from mendelian expectations; these departures are caused by the elimination, during individualization, of chromosomes contained within micronuclei that are formed in spermatids at the end of the second meiotic division. The effect of this chromosome loss is to increase the proportion of nullosomic gametes in the sperm pool.—The relative frequencies of XY-bearing and nullo-X, nullo-Y sperm in XY/O males were estimated from the volume measurements. Taking this estimate as a measure of the fertilizing population, it is possible to infer from the change in sex ratio over time following insemination, that XY-bearing sperm have an advantage of 1.5 over nullo-X, nullo-Y sperm in leaving the seminal receptacle of the female for fertilization of ova.     

Transmission ratio distortion of mouse t haplotypes is not a consequence of wild-type sperm degeneration

A mouse t-complex-specific DNA probe was used to determine the ratio of t-carrying and (+)-carrying sperm in epididymal, vas deferens, and postejaculatory sperm cell populations from heterozygous ($\text{+}\text{t}$) mice with transmission ratios of greater than 95%. No detectable degeneration of (+)-carrying sperm was observed. In this respect, mouse t haplotypes differ from Drosophila melanogaster SD chromosomes. High transmission of t haplotypes must be a consequence of differential transport and/or differential sperm function during the fertilization process itself.     

Further Characterization of Genetic Elements Associated with the Segregation Distorter Phenomenon in DROSOPHILA MELANOGASTER

Segregation Distorter, SD, associated with the second chromosome of Drosophila melanogaster, is known to cause sperm bearing the non-SD homologue to dysfunction in heterozygous males. In earlier studies, using different, independently derived, SD chromosomes, three major loci were identified as contributing to the distortion of segregation ratios in males. In this study the genetic components of the SD-5 chromosome have been the subjects of further investigation, and our findings offer the following information. Crossover analysis confirms the mapping of the Sd locus to a position distal to but closely linked with the genetic marker pr. Spontaneous and radiation-induced recombinational analyses and deficiency studies provide firm support to the notion that the Rsp (Responder) locus lies within the proximal heterochromatin of chromosome 2, between the genetic markers lt and rl and most likely in the heterochromatin of the right arm. The major focus of this study, however, has been on providing a better definition of the genetic properties of the Enhancer of SD [E(SD)]. Our findings place this locus within the region of the two most proximal essential genes in the heterochromatin of the left arm of chromosome 2. Moreover, our analysis reveals a probable association of the E(SD) locus with a meiotic drive independent of that caused by Sd.     

Defects in nuclear transport enhance Segregation Distortion

The Segregation Distorter (SD) system in Drosophila melanogaster causes the transmission of the SD chromosome at the expense of the SD⁺ chromosome. This occurs through a defect in sperm-specific chromatin condensation of the SD⁺-bearing spermatids of the SD/SD⁺ male. The Sd gene encodes a truncated form of RanGAP that is missing a nuclear export signal and is therefore trapped in the nucleus; normally RanGAP is found at the periphery of the nuclear membrane and is required for normal Ran-mediated nuclear transport. The presence of active RanGAP in the nucleus interferes with nuclear export and causes distortion. We show that mutations that affect nuclear import and export can enhance distortion in an SD background, thus verifying that the defect in nuclear transport is responsible for the unequal transmission of chromosomes. In addition, we identify several genes involved in chromatin condensation which also cause distortion in an SD background, opening the way to the dissection of the mechanism of segregation distortion.     

Experimental Population Genetics of Meiotic Drive Systems II. Accumulation of Genetic Modifiers of Segregation Distorter (SD) in Laboratory Populations

The accumulation of modifiers of the meiotic-drive locus Segregation Distorter (SD) in Drosophila melanogaster was monitored by measuring the changes in the mean and variance of drive strength (in terms of "make" value) that occur in laboratory populations when SD and SD+ chromosomes are in direct competition. The particular SD lines used are T(Y;2),SD translocations showing pseudo-Y drive. Four sets of population cages were analyzed. Two sets were monitored for changes in SD fitness and drive strength (presumed to be positively correlated) and analyzed for the presence of autosomal dominant or X-linked modifiers after long periods of time. The remaining two sets were made up of cages either made isogenic or variable for background genetic material, and these were used to test whether the rate of accumulation of modifiers was dependent on initial genetic variability.—Contrary to previous studies in which most suppression of SD action could apparently be attributed to a few dominantly acting modifiers of large effect, the conclusion here is that laboratory populations that are initially free of such major dominant loci evolve to suppress SD action by accumulating polygenic, recessive modifiers, each of small effect, and that much of the required genetic variability can be generated de novo by mutation. Possible explanations for these seemingly incompatible results and the evolutionary implications for SD are considered.     

Experimental Population Genetics of Meiotic Drive Systems I. Pseudo-Y Chromosomal Drive as a Means of Eliminating Cage Populations of DROSOPHILA MELANOGASTER

The experimental population genetics of Y-chromosome drive in Drosophila melanogaster is approximated by studying the behavior of T(Y;2),SD lines. These exhibit "pseudo-Y" drive through the effective coupling of the Y chromosome to the second chromosome meiotic drive locus, Segregation distorter (SD). T(Y;2),SD males consequently produce only male offspring. When such lines are allowed to compete against structurally normal SD⁺ flies in population cages, T(Y;2),SD males increase in frequency according to the dynamics of a simple haploid selection model until the cage population is eliminated as a result of a deficiency in the number of adult females. Cage population extinction generally occurs within about seven generations.—Several conclusions can be drawn from these competition cage studies:

(1) Fitness estimates for the T(Y;2),SD lines (relative to SD⁺ ) are generally in the range of 2–4, and these values are corroborated by independent estimates derived from studies of migration-selection equilibrium.

(2) Fitness estimates are unaffected by cage replication, sample time, or the starting frequency of T(Y;2),SD males, indicating that data from diverse cages can be legitimately pooled to give an overall fitness estimate.

(3) Partitioning of the T(Y;2),SD fitnesses into components of viability, fertility, and frequency of alternate segregation (Y + SD from X + SD⁺) suggests that most of the T(Y;2),SD advantage derives from the latter two components. Improvements in the system might involve increasing both the viability and the alternate segregation to increase the total fitness.

While pseudo-Y drive operates quite effectively against laboratory stocks, it is less successful in eliminating wild-type populations which are already segregating for suppressors of SD action. This observation suggests that further studies into the origin and rate of accumulation of suppressors of meiotic drive are needed before an overall assessment can be made of the potential of Y-chromosome drive as a tool for population control.

     

Germline mutations and variants in the succinate dehydrogenase genes in Cowden and Cowden-like syndromes

Individuals with PTEN mutations have Cowden syndrome (CS), associated with breast, thyroid, and endometrial neoplasias. Many more patients with features of CS, not meeting diagnostic criteria (termed CS-like), are evaluated by clinicians for CS-related cancer risk. Germline mutations in succinate dehydrogenase subunits SDHB-D cause pheochromocytoma-paraganglioma syndrome. One to five percent of SDHB/SDHD mutation carriers have renal cell or papillary thyroid carcinomas, which are also CS-related features. SDHB-D may be candidate susceptibility genes for some PTEN mutation-negative individuals with CS-like cancers. To address this hypothesis, germline SDHB-D mutation analysis in 375 PTEN mutation-negative CS/CS-like individuals was performed, followed by functional analysis of identified SDH mutations/variants. Of 375 PTEN mutation-negative CS/CS-like individuals, 74 (20%) had increased manganese superoxide dismutase (MnSOD) expression, a manifestation of mitochondrial dysfunction. Among these, 10 (13.5%) had germline mutations/variants in SDHB (n = 3) or SDHD (7), not found in 700 controls (p < 0.001). Compared to PTEN mutation-positive CS/CS-like individuals, those with SDH mutations/variants were enriched for carcinomas of the female breast (6/9 SDH versus 30/107 PTEN, p < 0.001), thyroid (5/10 versus 15/106, p < 0.001), and kidney (2/10 versus 4/230, p = 0.026). In the absence of PTEN alteration, CS/CS-like-related SDH mutations/variants show increased phosphorylation of AKT and/or MAPK, downstream manifestations of PTEN dysfunction. Germline SDH mutations/variants occur in a subset of PTEN mutation-negative CS/CS-like individuals and are associated with increased frequencies of breast, thyroid, and renal cancers beyond those conferred by germline PTEN mutations. SDH testing should be considered for germline PTEN mutation-negative CS/CS-like individuals, especially in the setting of breast, thyroid, and/or renal cancers.     

Chromosomal Control of Fertility in DROSOPHILA MELANOGASTER . I. Rescue of T(Y;A)/bb Male Sterility by Chromosome Rearrangement

Analysis of X-ray-induced deletions in the Segregation Distorter (SD) chromosome, SD-5, revealed that this chromosome had a gene proximal to lt in the centric heterochromatin of 2L that strongly enhanced the meiotic drive caused by the SD chromosome. This Enhancer of Segregation Distortion [E(SD)] locus had not been characterized in earlier studies of SD chromosomes because it cannot be readily separated by recombination from the Responder (Rsp) locus in the proximal heterochromatin of 2R.—To determine whether E(SD) is a general component of all SD chromosomes and to examine further its effects on distortion, we produced deletions of E(SD) in three additional SD chromosomes. Analysis of these deletions leads to the following conclusions: (1) along with Sd and Rsp, E(SD) is common to all SD chromosomes; (2) the E(SD) allele on each SD chromosome enhances distortion by the same amount, which indicates that allelic variation at the E(SD) locus is not responsible for the different drive strengths seen among SD chromosomes; (3) E(SD) causes very little or no distortion by itself in the absence of Sd; (4) E(SD), like Sd, acts in a dosage-dependent manner; (5) E(SD) exerts its effect in cis or trans to Sd; and (6) if E(SD)⁺ exists, its function is not related to SD.     

Suppressor Systems of Segregation Distorter (SD) Chromosomes in Natural Populations of DROSOPHILA MELANOGASTER

Several natural populations of D. melanogaster were investigated for the presence (or absence) of the Segregation Distorter ( SD) chromosomes and their suppressor systems. The SD chromosomes were found, at frequencies of a few percent, in two independent samples taken in different years from a Raleigh, North Carolina, population, whereas no SD chromosomes were found in samples collected from several populations in Texas. The populations in these localities were found to contain suppressor X chromosomes in high frequencies (75% or higher). They also contained relatively low frequencies of partial suppressor or insensitive second chromosomes of varying degrees, but completely insensitive second chromosomes were practically absent in all populations examined. The frequencies of suppressor X chromosomes, as well as those of the partially insensitive or suppressor second chromosomes, were the same among the populations investigated. This suggests the possibility that the development of a suppressor system of SD in a population could be independent of the presence of an SD chromosome. Segregation distortion appeared to be occurring in natural genetic backgrounds, but the degree of distortion varied among males of different genotypes. There were many instances in which the SD chromosomes showed transmission frequencies from their heterozygous male parents that were smaller than 0.6 and, in several cases, even smaller than 0.5. The presence of a recessive suppressor, or suppressors, of SD in natural populations was suggested.     

The effects of the photoperiod-insensitive alleles,se13, hd1 and ghd7, on yield components in rice

Flowering time is closely associated with grain yield in rice (Oryza sativa L.). In temperate regions, seasonal changes in day length (known as the photoperiod) are an important environmental cue for floral initiation. The timing of flowering is important not only for successful reproduction, but also for determining the ideal balance between vegetative growth and reproductive growth duration. Recent molecular genetics studies have revealed key flowering time genes responsible for photoperiod sensitivity. In this study, we investigated the effect of three recessive photoperiod-insensitive alleles, se13, hd1 and ghd7, on yield components in rice under Ehd1-deficient genetic background conditions to ensure vegetative growth of each line. We found that se13-bearing plants had fewer panicles, hd1-bearing plants showed decreased grain-filling percentage, and ghd7-bearing plants appeared to have fewer grains per panicle and fewer secondary branches. Our results indicate that the pleiotropic effects of photoperiod-insensitive genes on yield components are independent of short vegetative growth. This will provide critical information which can be used to create photoperiod-insensitive varieties that can be adapted to a wide range of latitudes.